The XA has multi-channel capability while the scd is stereo only and if I can remember neither has balanced outs. I believe there is a different DAC architecture in the XA as well. The scd has better build quality in my opinion and was Sony's statement product along with the SCD-1. If you purchse the scd, and can afford it, have the superclock 3 and superclock power supply installed, it really improves the player on both redbook and SACD.
Chuck
Chuck